Project Summary/Abstract Animal-assisted interventions (AAIs) vary widely in terms of the techniques, species, and populations involved. In addition, the mechanisms of action that are common across different types of AAIs are not well understood. Elucidating the processes through which AAIs produce improvements will be critical to establishing the credibility and efficacy of AAIs, as well as to efforts to leverage and enhance the benefits. The proposed project will evaluate a mechanism of action that may underpin AAIs for individuals with depression, anxiety, and trauma-related symptoms. Specifically, this project will investigate how emotional states transmit between people and dogs (the species most commonly involved in AAIs). First, a set of high-quality stimulus videos of canine emotional expressions will be developed. The Facial Action Coding System and Body Action and Posture Coding System will then be used to evaluate whether humans mimic the emotional expressions of dogs, and whether that mimicry is involved in improvements in mood. Finally, canine mimicry of human emotional expressions will be evaluated using the Dog Facial Action Coding System and a canine ethogram. Evidence for emotional contagion between humans and dogs would constitute a significant step in efforts to establish the efficacy of AAIs, inform the development of guidelines for practice, provide a method for selecting animals for participation in AAIs, and underscore the importance of protecting the happiness and well-being of animals involved in AAIs.